Dometic Coolmatic RPD 218 Modification

Back in April 2019, I did a quick review of this fridge and because I had just installed it, and we were going towards the cooler season, I had not much to tell other than the good electrical consumption and the different built materials, from what was advertised where I purchased it.

Now that I had the pleasure to use this fridge in a very hot summer, unfortunately, I had to modify the fridge so that it can work in temperatures above 35 °C as advertised. After all, this is a class T fridge and therefore it has to work fine in temperatures up to 43 °C. Some people confuse the class rating of the fridge with the Australian Energy rating. They are two completely different ratings. The first is related to the fridge environment working temperature, were the latter is related to the working efficiency of the fridge. While they are somewhat related, in this article I’ll concentrate on the working environment of the fridge.

Please note that this is a long post, but it does cover everything I have learned about this fridge. The RPD 218 does work (to some extent) perfectly fine to air temperatures up to 28 – 30 °C. After that, while still working, as it still refrigerate, unfortunately it stays on pretty much all the time and it just doesn’t cope with higher temperatures.. This is not a fault. But it does create problems.. Perhaps I better start from the beginning.

I also have to acknowledge that this is the situation with this fridge, my fridge, and that perhaps other people with the same fridge have may experience a slight different situation, but gathering from what I read in different forums, I sincerely doubt it.

Since day one, my fridge did work as follows: Freezer -20 °C (always fine) even when the air temperature did reach 41 °C in December. Fridge always floating between 0 – and 10 °C and then sitting on 8 °C when December came. As you can see the fridge as not been right since day one. But I only started to notice this, since I used a thermometer and left it inside the fridge permanently.

So I can definitely say, that while the Freezer it is a class T, the fridge part of the RPD 218, on the other hand, is not! Back in December, after realising that the fridge was not working as advertised, I started to look for two things on the web. Firstly if someone else has had the same problem and secondly if a permanent solution was available. I knew that the fridge, while not cooling enough was working fine, as the freezer had no problems.

The first thing I did find online was that pretty much everybody else has a similar problem, as in that the fridge was either too cold or too hot and that the cycling was not really working. Several solutions were offered to these people, ranging from calling Dometic and having parts replaced to people suggesting turning the fridge upside down for a night.. You got to love Internet. 😂 To see all the finding, just Google “Dometic RPD 218 problems” and you can see the list…

The most common complaint, seemed to be that the fridge was not cycling, as it stayed on all the time. That was one of my problem as well..

Amongst all the suggestion there were a couple claiming that after having replaced the original  fan mounted on the top of the little condenser with a more powerful one, the fridge did run just fine. Could be that those two people had found the problem? I didn’t want to believe that Dometic had mistakenly used a fan not powerful enough for the fridge to miserably fail in temperature above 30 °C. Come on, they must have done extensive testing before releasing this new model.

I decide to dig peeper and to understand a bit better what was actually happening to my fridge. By looking at the condenser and the original fan, it only took two day to realise that it always works at ambient temperature. So if you have 30 °C inside your vehicle, the condenser will be also working at 30 °C. No matter how big or powerful the fan is, it will always cool down to ambient temperature. 30 – 34 °C in the day and 24 -25 °C at night. This at least does explain why in summer the fridge compressor does not stop. If the air temperature inside your vehicle is too hot (by my testing anything above 30 °C), then your RPD 218 will stay on most of the day, not at full speed, but definitely on. Once the temperature passes 35 °C the fan start to go faster, not sure to what end as the condenser now is cooled by warmer air so the speed of the fan makes no real difference. Unless you find a way to have cooler air available, the size or speed of the fan is completely irrelevant. More hot air does not dissipate heat any quicker.

One day in December, the temperature did manage to hit 43 °C, and I was able to take various temperature from different location of the fridge:

• Air temperature 43 °C  <—- Limit temperature for a class T fridge
• Freezer Temperature -18 °C <— Perfect temperature for a freezer
• Fridge Temperature 10 °C <— Not safe or right for a fridge
• Condenser temperature 43 °C  <—- Note that it is the same as the air temperature
• Compressor temperature 74 °C <— Wrong temperature for this compressor

Armed with this data, I decided to look at the manufacturer specs. The compressor is made by the famous Chinese company Wanbao and unfortunately there is no much data available online, but the consensus is that these small compressor last the longest if they run at temperature of 50 – 55 °C. I couldn’t find any confirmation of this but it did seem right as Wanbao does sell these unit with an ST class rating (not T as advertised by Dometic, but it supposed to become class T with the addition of the fan). The compressor, based on my temperature data taken within two weeks also seem to confirm that around those compressor temperatures, the fridge does cycle and work as intended.

Lets put few numbers together.. On the hottest day my compressor was running at 74 °C and was not able to cool the fridge section (10 °C). Somehow I need it to find the way to test the compressor capabilities at a lower temperature. Most domestic fridge have a plastic jacket above the compressor where the defrosted water does evaporate using the heat from the compressor. While this is not the same problem (as the RPD does have a drain hose) it still gave me an idea of how to cool the compressor. In the second picture you can see how with a simple little piece of aluminium angle bar, I was able to attach a second fan pointing directly at the compressor. That did work but not enough. The compressor was now running at 64 °C with an air temp of 35 °C and a fridge temp of 8 °C.

In the third picture, you can see that the compressor now has started to look like a porcupine. Those are 50 heatsink attached with thermal double side tape from Jaycar (HH8514). Now at an air temperature of 34 °C the compressor was running at a range between 38 °C and 47 °C and the fridge finally reaching a decent 2 °C at the lowest setting. The fan was obviously on. Three days of testing did confirm the fact that now both Freezer and fridge are running at the correct temperature for a T class fridge at an air temperature of 35 °C.  Theoretically at an air temperature of 43 °C I should expect the compressor to run at 47 – 56 °C and not any more at 74 °C.

Finally I did connect the secondary fan by splicing the original cable that goes to the condenser fan. This was just a temporary solution as because the fan is voltage controlled by the computer (Turbo mode), this mean that two fan will spin slower than a single one. So the proper solution is to connect the secondary fan to the 24 volts input and be controlled by a  35 °C N.O. thermostatic switch. This solution will allow both fan to run at their intended speed and CFM capacity. While on the computer (fridge controller) subject, it also pays to remember that because directly mounted on the side of the compressor, it will benefit as well by having electronic components running cooler and lasting longer.

Lastly as recommended by Dometic I would suggest to also have a fan located at the bottom of the refrigerator where the fresh (cooler) air intake as per page 12 of the manual. Only then, you can say that your RPD 218 is truly a class T rated fridge. The fridge is now a fraction noisier because of the extra fan, but that does only happen in the day not at night, as the fridge now does cycle beautifully as intended. I’m sure that there are other ways to keep the compressor cooler and if you have come up with something even better, I would like to hear it…

As usual, any questions, are welcome..